Nondispersive analyzers are analyzers which provide discrete radiation paths from a source to one or more means for detecting the intensity of radiation. One of these paths is through the sample to be analyzed as well as through an isolated fixed quantity of the analyte gas, and another of these paths is through the sample to be analyzed only. Therefore, that part of the radiation which is at the wavelengths absorbed by the analyte gas is removed from one path, but not the other, resulting in a difference in the outputs from the energy detecting means associated with these paths. Thus, it is only when the sample contains the analyte gas that the difference between the intensities of the radiation along the two paths will be reduced. Other absorbing compounds affect both detectors equally.
In order to ensure that error is not introduced by reason of some variation between radiation sources, a single source is usually used to generate the beams of radiation over both paths. Thus, a beam splitter or chopper is required. Analyzers of this sort may be relatively large. Further, appreciable energy is consumed, often of the order of tens of watts. This is required to power the source, to operate the chopper motor for the beam splitter, and to power the measuring circuits.
This invention provides a nondispersive analyzer which is relatively small and light in weight, and which does not require either a beam splitter or a mechanical chopper. It requires very little power, approximately one watt, and that only intermittently. It can therefore be operated for several weeks on the energy stored in a lightweight battery. It uses an elliptical reflector to focus light from its energy source.
Elliptical cylinder and ellipsoidal reflectors are commonly used in optics to focus light. Such reflectors are frequently used in measuring devices. Thus U.S. Pat.No. 3,266,313 (Litterest) shows a temperature measuring device where the object whose temperature is to be measured (a wire) is at one focus of an ellipse and the detector is at another focus. U.S. Pat. No. 4,810,658 (Shanks) shows in FIG. 4a a system where a liquid sample in contact with a solid wave guide is placed at one focus of an ellipse and a light source at the other focus.
Partially elliptical or ellipsoidal mirrors are shown in Canadian Patents 1,126,977 (Hogg) and 1,127,867 (Brunsting) for particle counters. The sample and light source are located at one focus of the ellipse. A detector is located either on the axis of the two foci (in Brunsteig) or is reflected off this axis by a mirror (in Hogg).
C.P. 1,228,748 (Oetliker) shows a variety of light guiding designs for various purposes using ellipsoids. In some of the designs the light source and the sample are at one focus of an ellipsoid and a detector is at the other focus. In other designs, a light source is at one focus and a specimen to be treated by light (as for example in a chemical process) is at the second focus.
Elliptical or ellipsoidal reflectors are not common in spectrometry. Three patents of Oehler, U.S. Pat. Nos. 4,557,603; 4,657,397 and 4,740,086 and one of Miyatake (U. S. Pat. No. 4,808,825) disclose infrared spectrophotometers. However, such spectrophotometers are not of the non-dispersive type, and have only one gas cell which is traversed by the light. They do not assist in the design of a non-dispersive gas analyzer, where light rays must pass through several gas cells to give a comparative measurement.